1. Field of the Invention
The present invention relates to a silicon carbide (SiC) semiconductor device and a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
A silicon carbide semiconductor has a wider band gap compared to that of a silicon (Si) semiconductor and has therefore high breakdown electric field strength. “On-resistivity”, which is resistivity in the conduction state, is inversely proportional to the cube of the breakdown electric field strength and therefore, in silicon carbide semiconductors (a four-layer-cycle hexagonal crystal: 4H—SiC) that are widely used and called 4-H types, the on-resistivity can be suppressed to be several-hundredths of that of a silicon semiconductor.
With its property of high thermal conductivity that facilitates heat dissipation in addition to the above, the silicon carbide semiconductor is therefore expected to become a next-generation low-loss power semiconductor device. Silicon carbide semiconductor devices having various structures have been developed using the silicon carbide semiconductor such as, for example, a Schottky barrier diode, a MOSFET (an insulated gate field-effect transistor), a PN diode, an IGBT (an insulated gate bipolar transistor), and a GTO (a gate turnoff thyristor).
On the other hand, concerning an ohmic electrode important in fabricating a silicon carbide semiconductor device, although it is clear that for an n-type region, excellent ohmic contact can be formed by using nickel (Ni) as the material of the ohmic electrode, for a p-type region, the material and a structure to form the ohmic contact are still under study. A multi-layered film of aluminum (Al) and Ni has been proposed as an example of an ohmic electrode in the p-type region (see, Ito, Kazuhiro, et al, “Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H—SiC”, Journal of ELECTRONIC MATERIALS, Vol. 37, No. 11, 2008, pp. 1674-1680).
An ohmic electrode is generally formed by forming a metal film on a silicon carbide semiconductor layer and thereafter, performing annealing under reduced pressure or in an inert gas atmosphere to produce a metal silicide. When a silicon carbide semiconductor device is produced, the annealing is generally performed at about 1000 degrees C. However, when annealing is performed for a multi-layered film of different metals such as the above film of Ni and Al, an alloy is produced between the metals without the formation of metal silicide between the silicon carbide semiconductor layer and the metal. A problem therefore arises in that this does not function as an ohmic electrode.
A rapid annealing (RTA: Rapid Thermal Annealing) method is a method of efficiently producing a metal silicide. The RTA method is based on a mechanism that uses an infrared ray lamp to heat the substrate by a high heating rate, where the object to be heated is generally placed on a carbon susceptor because this annealing also efficiently heats materials having low infrared ray emissivity such as, for example, metal and synthetic quartz materials.
When this mechanism is applied to the formation of a silicon carbide semiconductor device, a majority of the infrared rays from a halogen lamp used as the infrared ray lamp are absorbed by the carbon susceptor and not by the silicon carbide substrate because the silicon carbide semiconductor substrate does not absorb near-infrared rays of wavelengths up to about 2.5 μm. The silicon carbide substrate is heated by thermal conduction due to the contact with the susceptor. Temperature distributions occur on the front face of the silicon carbide substrate and at portions not in contact with the susceptor depending on the shape of the substrate and are the reason no metal silicide is formed.
This uneven formation of the metal silicide causes variance in the contact resistivity and variance in the properties and the reliability of the device.